1. Field of the Invention
This invention relates to improvements in heterojunction transistors and methods for making same, and more particularly to an improved heterojunction transistor which has controllable base-emitter diode cut-in voltages, controllable offset voltages, and increased gain than predicted by classical transistor theories, and to a method for making same.
2. Description of the Prior Art
In 1950, U.S. Pat. No. 2,569,347 issued to Shockley, a wide bandgap emitter heterojunction transistor with improved gain and performance was disclosed. Since then, much work has been expended to realize the full theoretical potential of this type transistor, but without total success.
Today, it is known that semiconductor bandgap energy is the energy required to create a hole-electron pair by exciting a mobile hole into the valence band and a mobile electron into the conduction band. Two methods of creating mobile carriers in the respective bands are by creating mobile hole-electron pairs and by introducing impurity atoms which create a mobile carrier in one band with a residue ionized lattice atom. The mobile carriers created by these two methods are distinctly different.
By the hole-electron pair method, a mobile electron is excited into the conduction band and a corresdponding mobile hole into the valence band, with a corresponding increase in potential energy (less negative lattice energy). The mobile electrons and holes in this case always exist in pairs, termed excitons.
By the impurity atom introduction method, impurity atoms are introduced near one of the two bands, to create mobile carriers at very low temperatures prior to exciton formation. For instance, donor impurities are located near the conduction band and acceptor impurities are located near the valence band. In silicon, for example, acceptor impurities are trivalent atoms, like boron or gallium, and donor impurities are pentavalent atoms, like phosphorous or arsenic. Ionized mobile carriers never occur in pairs between the conduction and valence bands, but are always coupled to the ionized impurity atoms. The pairs of ionized impurity atoms and ionized mobile carriers are termed polarons.
The chemostatic potential of a semiconductor region is related to the polaron/exciton ratio for usable energy ranges. Specifically, the chemostatic potential energy is EQU kTln(N/n.sub.i)
for N&gt;&gt;n.sub.i, where N is the net ionized impurity concentration and n.sub.i is the intrinsic carrier concentration. The intrinsic carrier concentration is, among other things, a measure of the bandgap energy. In steady-state, excitons are generated and recombined at identical rates, the recombination rate. The inverse of the recombination rate, normalized with respect to the exciton concentration, is the exciton lifetime, or the statistical duration of an exciton before hole-electron pair annihilation. The inverse of the exciton lifetime gradient is the recombination velocity, which is the measure of the spatial recombination rate variation. Diffusion can only exist in the presence of a non-zero recombination velocity, that is, a spatially variable recombination rate, and the propensity to diffuse is enhanced by increasing recombination velocity.
The total chemostatic potential energy is a measure of the potential energy available to support the transport of minority carriers by diffusion. Greater chemostatic potential energy magnitude manifests larger potential recombination velocity, which can result in larger diffusion currents. Mobile electron diffusion is favored for large positive chemostatic potential energy, while mobile hole diffusion is favored for large negative chemostatic potential energy. Thus, in a bipolar transistor, it is highly desirable to have a very large chemostatic potential energy magnitude in the base region to enhance minority carrier transport through the base. Although the chemostatic potential is a relative measure of polarons and excitons, a secondary diffusion effect depends on the absolute value of the exciton concentration. The electrochemical potential drop associated with minority carrier diffusion through the base decreases with increasing exciton concentration. Therefore, it is desirable to form the base region with a narrow bandgap material, that is, a material with a high intrinsic carrier concentration, and high impurity doping with the relative impurity/intrinsic carrier concentration large. For an NPN transistor, it is desirable to have a high acceptor doping concentration in the base to create a negative chemostatic potential energy, resulting in a large recombination velocity enhancing a large gradient of mobile electrons transiting the base with a high diffusion velocity.
A heavily acceptor doped region has the additional advantage of reducing the base de-biasing resistance. This increases the total minority carrier flux through the base, and increases the transistor current density capability. Unfortunately, at high impurity atom concentrations, the band structure is altered such that bandgap narrowing occurs and the chemostatic potential self-limits. Furthermore, increasing the magnitude of the base chemostatic potential tends to reduce minority carrier injection into the base, which degrades transistor performance. The relative minority carrier injection across a junction is related to the relative chemostatic potential magnitude. This injection efficiency can be improved by increasing the magnitude of the emitter chemostatic potential (opposite polarity of the base chemostatic potential). This effect is limited by emitter bandgap narrowing at high polaron concentrations.
Preferential minority carrier injection is due to the relative chemostatic potential magnitude with higher minority carrier injection into neutral region of lower chemostatic potential magnitude. Minority carrier diffusion is favored in regions of high recombination velocity, presuming a region of high chemostatic potential magnitude with the proper boundary conditions. In order to obtain a high diffusion flux it is desirable to have a low recombination velocity at the boundary of the injection source (the emitter) and a high recombination velocity at the collector boundary. In a transistor biased in the active region, the reverse biased junction at the collector-base boundary provides the high boundary recombination velocity required due to the large potential for minority carriers. It is desirable to establish a vanishing recombination velocity in the emitter to eliminate any diffusion current into the emitter and to provide all emitter transport by majority carrier drift. Therefore, it appears desirable to have an emitter void of recombination, commensurate with a region of high chemostatic potential magnitude.
In U.S. Pat. No. 2,569,347 mentioned above, two methods of enhancing the emitter chemostatic potential are predicted, first, increased impurity atom concentration, i.e. increased polaron concentration, and second, increased bandgap energy, i.e. reduced exciton concentration. Substantially reduced emitter diffusion current presumes a combination of high donor doping concentration or wide bandgap energy along with low boundary recombination velocity at the emitter contact, which is characteristic of a low exciton concentration. Thus, the ideal transistor is characterized by a relatively large emitter/base chemostatic potential magnitude ratio, with a low recombination rate at the emitter boundary and a high recombination rate at the collector boundary. The emitter has a moderate polaron concentration and a very low exciton concentration due to a wide bandgap energy. The base has a narrower bandgap energy and a high acceptor doping concentration to maintain a lower chemostatic potential magnitude relative to the emitter. Thus, minority carrier injection into the emitter and minority carrier recombination in the emitter are both substantially reduced. This provides a very low emitter recombination velocity which favors majority carrier transport by drift. For a wide bandgap emitter heterojunction transistor, majority carrier transport in the emitter is favored, minority carrier injection into the emitter is substantially reduced, minority carrier injection into the base is enhanced, minority carrier diffusion in the base region is enhanced, and ohmic base debiasing is reduced. This should result in a large emitter-collector electron flux for an NPN transistor, with a small base hole flux which allows for a high gain device with good switching characteristics. Compound semiconductor devices, such as AlGaAs/GaAs transistors, operate on this principle. Unfortunately, full performance has not been realized to date.